Vacuum cleaners are often evaluated primarily in terms of their input power, whereby a high input power raises the expectation that this will translate into a strong suction power. Under various operating conditions and the associated restriction of the suction, that is to say, for example, due to differences in the floor being vacuumed, especially in the case of carpeting of differing densities, the electric input power of a vacuum cleaner does not remain constant but rather drops steadily from the “open hose” state (without restriction) to the complete closure of the suction hose. However, the air watt P2—as the product of negative pressure and flow rate—has its maximum approximately in the middle between the maximum flow rate and thus the maximum electric input power, and the flow rate of zero and thus the minimum electric input power. This means that, due to the characteristic curve of the fan and of the motor, within the range of the maximum achievable air watt (approximately in the middle of the characteristic curve), it is no longer possible for the initial, maximum electric input power to be obtained from the mains and converted, but rather, only a much lower electric input power is available to be converted into air watt.
Since the input power of the drive unit of the vacuum cleaner fan—referred to below as the “input power of the vacuum cleaner” for short or simply as the “input power”—could become increasingly relevant when it comes to meeting stipulations for complying with a given energy efficiency class for electric household appliances and thus also for vacuum cleaners, it is normally not an option to increase the input power in order to compensate for the diminishing air watt.
Moreover, for reasons having to do with the system, as the dust bag fills up, the air watt available at the floor tool decreases because the pressure loss inside the dust bag increases with the filling level. Due to a likewise diminishing flow rate through the floor tool or through a suction hose connected to the floor tool, and due to fact that the input power of a drive unit of the vacuum cleaner fan depends on the flow rate, the input power drops even more, so that in the final analysis, only a small amount of air watt is available at the floor tool.
European patent application EP 1997417 A2 describes the suction pressure on the basis of a combination of the power measurement and the rotational speed of a reluctance motor.
One procedure with vacuum cleaners is to ascertain the drawn current and to limit it to a maximum value in order to comply with restrictions pertaining to the maximum current input in supply networks with limited current input. Japanese patent application JP 5 023273 A, for instance, describes such a method. US patent application 2007/136980 A1 and Japanese patent application JP 11221180 A are likewise aimed at limiting or regulating the motor current. Here, however, voltage fluctuations are not taken into account. Such voltage fluctuations have different causes:                As a rule, electric devices are approved for a wide voltage range. Thus, for example, the same device can be used in Japan at a rated voltage of 100 volts and in the United States at a rated voltage of 120 volts. Since the voltage decreases quadratically, this leads to a 44% higher power input in the United States.        Voltage fluctuations occur in the voltage networks as a function of the total consumption by users; here, 10% is permissible.        Switching on loads that entail a high input power, which includes vacuum cleaners, results in a load-dependent voltage drop.        
All in all, due to the possible voltage fluctuations, the desired suction power can fluctuate by up to 50%, even in vacuum cleaners whose motor current is limited or regulated.